This invention relates to the production and stimulation of oil wells and more particularly, to a method of determining the length change of a string of tubing in an inclined well.
Gas wells and flowing oil wells are usually completed and treated through a string of tubing and a packer. Changes in temperature and pressure during stimulation and production of a well usually result in changes in tubing length, tubing stress, and packer load. These changes in tubing length and stress are quite substantial especially in deep high temperature, high pressure wells. Costly failure occurs if the stresses exceed the tubing mechanical strength, or if the seal length is inadequate to compensate for the length change. If the fluid pressure inside the tubing is much greater than that outside, the tubing may buckle helically, even if there is packer-to-tubing tension.
The forces acting on a tubing string which undergoes changes in temperature and in pressure, and a study of helical buckling is contained in "Helical Buckling of Tubing Sealed in Packers," A. Lubinski, W. S. Althouse and J. L. Logan, Petroleum Transactions June 1962, pp. 655-670. This study is extended to combination completions having varying tubing and/or casing sizes in "Movement, Forces and Stresses Associated With Combination Tubing Strings Sealed in Packers," D. J. Hammerlindl, February, 1977, J. of Pet. Tech., pp. 195-208. "Tubing Movement, Forces, and Stresses in Dual Flow Assembly Installations," Kenneth S. Durham, SPE 9265, Paper presented at the 55th Annual Fall Technical Conference of the Society of Petroleum Engineers of AIME, Dallas, Texas, Sept. 21-24, 1980, extends the study to situations involving dual flow assembly installations.
The present invention is an improvement on the techniques discussed in the foregoing prior art. More particularly, the present invention is an improvement which can be used in sharply inclined wells where buckling may or may not occur, depending on the forces which are applied to the tubing string. The presence or absence of buckling is an important component of length change. The present invention provides an improvement in the accuracy in the determination of length change because it determines whether or not buckling has occurred.